As known, it is now common in bicycles to use disc brakes. Such brakes are indeed often preferred to conventional brakes of different design since they ensure a high braking force and are less subject to problems caused by mud or water.
Typically, a disc brake comprises a caliper fixed to the frame of the bicycle and a brake disc mounted on the hub of the wheel. Inside the caliper there are two or four opposite brake pads. The brake disc rotates inside the space defined between the opposite pads. By actuating the brake lever, the pads are brought towards the brake disc, generating friction on the brake disc and, consequently, braking the wheel.
The brake disc comprises a braking track configured to cooperate with pads and a radially inner annular portion for coupling with the hub.
The braking track is made of a material that ensures good braking characteristics, for example it can be made from steel.
The brake disc can be made in a single piece or, in order to reduce the weight thereof, it can be made in two components.
In this case, the brake disc comprises a first component having the braking track and a second component having the radially inner annular portion for coupling with the hub.
The first component is made of a first material that ensures good braking characteristics, like for example steel, whereas the second component is made of a lighter second material, like for example aluminum or light alloys.
The second component has a plurality of radially outer portions for connecting to the first component at a respective plurality of connection portions of the first component.
In the jargon, the second component is called spider, whereas the first component is simply indicated with the expression “braking track”.
The connection between the radially outer connection portions of the second component and the connection portions of the first component can be fixed or, alternatively, can be such as to allow a relative radial and/or axial clearance between the first and the second component.
Such relative clearance is useful to stop the thermal dilations of the first component that occur during use (by the friction generated by the contact between pads and braking track) creating undesired stresses at the connection areas between first component and second component.
As far as the coupling with the hub is concerned, the radially inner annular portion of the brake disc (which is made in the second component, in the case of a brake disc made in two components) is provided with a grooved radially inner surface (i.e. a radially inner surface that extends longitudinally and is provided with longitudinal grooves), which is mounted on a matching grooved radially outer surface (i.e. a radially outer surface that extends longitudinally and is provided with longitudinal grooves matching those of the radially inner surface) of a portion of the hub, as is disclosed in patent application EP 1932753 to the same Applicant.
EP 1932753 also discloses a lock nut, which is screwed onto the hub until it comes into axial abutment on the radially inner annular portion of the brake disc, so as to define the axial portion of the brake disc on the hub.
The Applicant has observed that, in the case of a brake disc made in two components, it is necessary for the connection between the radially outer connection portions of the second component and the connection portions of the first component to be extremely reliable, so that a certain braking quality is guaranteed as much as possible.
Damage to or breaking of the connections between first and second component could result in great limitations in braking quality, even to the point of braking itself being impossible, with the obvious serious consequences in terms of safety.
The above becomes even more important, if such a thing is possible, in the field of racing bicycles.
The problem at the basis of the present invention is to make a brake disc for a bicycle that, as well as having excellent features of strength and lightness, ensures extreme certainty of braking over time.